why "L'année prochaine" is femenine?

Cesar - the noun ‘ année ‘ is (always) feminine. 

Perhaps you are confusing ‘ année’ with the noun ‘ an ‘, which is masculine, but not appropriate to use in the sentence ? 

As a general rule, asking ‘ why ‘ with noun genders, is a futile exercise. They just need to be learnt/acquired through exposure, notwithstanding that some clues may be helpful in guessing.

 https://www.wordreference.com/fren/année

 Identifying the gender of nouns by some specific word endings in French